There's a Mystery Glitch in the Pipe Organ

At Reuter Organ Co. we built real pipe organs. My position was director of production and senior engineer. I had a problem once that I had to solve from 600 miles away. As a musician, I can use my musical skills to analyze various mechanical sounds scientifically. As a licensed amateur radio operator, my electronic skills help to make some problems relatively easy to solve. This particular problem was not so easy.

I received a call from our area representative who told me that the large pipe organ console we had just built and installed was blowing fuses in the three large DC power supplies. The fuses would blow during only about 5 percent of the power-up cycles and all three supplies would blow at the same time.

These supplies delivered power to three independently separate circuits. The representative revealed that it was all or none and whenever it happened, there was a short mild "burp" sound. I asked him to check for any shorts or any other abnormal circuit conditions that would cause over current conditions -- there were none. Everything was normal except for the few times that all three fuses would blow randomly, all at the same time.

Since this console was located almost 600 miles away, I decided to set up the exact circuit conditions in my lab with the one 50 amp and two 35 amp 12V DC supplies delivering power to three similar loads. I connected the supplies to a similar console. I starting latching relays to that in the console that activates the console lights, power supplies, and main air blower controller located in the utility room.

Everything operated correctly with no blown fuses. Since the problem occurred randomly about 5 percent of the time, I continued to cycle the system on and off many times. Finally, I heard a mild burp and all three supplies blew their fuses. I had to repeat this experiment many times to locate the burp sound again. It came from the latching relay.

Now, I could listen for the musical pitch, a low B, from the relay to identify the frequency and duration of the burp, which was 60 Hz for about 1/4 second. But why did it buzz only about 5 percent of the time? Putting a variac on the system, I could replicate the buzz more often with lower voltages. I determined that the spring and armature of the relay mechanically resonated at about 60 Hz and that the inrush current from the largest supply was sufficient to drop the supply voltage just enough to cause the relay armature to drop out for half a cycle.

Once that happened only during the short voltage peak of the AC cycle, the armature would oscillate at 60 Hz, thus operating as a mechanical rectifier causing pulsating 120V DC to continue to the three supplies. While power transformer primaries appear as relatively high impedance devices under little load to AC, they are a virtual dead short at DC. That is why all three supplies always blew fuses at the same time during those 5 percent random times. We fixed the problem by changing out the relay to another brand with an armature/spring resonance at a different frequency than 60 Hz. We also now use switching supplies.

This entry was submitted by Robert J. Vaughan and edited by Rob Spiegel.

Robert J. Vaughan received a BMus in Pipe Organ Performance in 1965 from Bethany College. He built a large three-manual pipe organ just out of college and rebuilt several other pipe organs until he was hired by Reuter Organ Co. in 1969 as a draftsman. Having a keen interest in music, structural engineering, and amateur radio electronics, Robert integrated all three disciplines into his final position with the company as director of production from the late 80s until retirement in 2008. He still does contract design work for the company and likes to restore old wooden tube radios from the 1930s.

In the software world, SAP does the same thing. They are very specific about what hardware, system software and middleware can be used with their ERP products. This is a real pain, until you have a problem that needs fixing. With a certified installation, they have the test fixtures for all valid combinations and can run in the same environment that you have. It allows them to give a very high SLA.

Not just a great example of patient and methodical troubleshooting - but also an example of how you can combine your areas of interest in your profession. While I have been a test engineer for years, I also have a passion for horses. I have a small business where my husband and I develop portable trail obstacles for horses. We often combine our mechanical engineering skills to solve problems with our obstacles and are currently developing some obstacles that are PIC controlled. We have a water obstacle that we eventually plan to have activated by a motion sensor. That's the awesome thing about engineering - you can bring it into so many different areas and work on those that specifically interest you - just as the author of this very interesting article has shown...

Good point, Naperlou, One thing a really like about this solution is how those who built the pipe organ worked to replicate the problem 600 miles away. This is reminiscent of how the Apollo 13 problem was solved. Those in Houston tried to replicate the materials those in the spacecraft has so they could use those materials to solve the problem.

Beth, I agree with you. That was an interesting story and a very interesting problem. There is no way to teach such skills. You just have to work with the equipment and understand it at many levels to find a solution.

Nice story and an on-going lesson for up-and-coming engineers that despite the complexities of today's products and tool platforms, patience and persistence as well as a thirst for curiousity and an eye for creative problem solving are still the tried and true foundational skills for good engineers. Thanks for sharing.

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